In many chemical analyses a comparative technique is used to determine properties of a sample. This involves the measurement of one or more characteristics of a plurality of standards to produce a calibration curve. For example in atomic absorption spectroscopy a series of standards containing a known range of concentrations of a given element are measured to determine their absorption and a graph of absorption against concentration is plotted. The absorption of an unknown sample is then measured and the concentration of the element to be detected is then determined using the calibration curve. It is generally assumed that at least at lower concentrations the calibration curve will be linear and will pass through the origin. Various techniques can be used for constructing the best straight line calibration curve from the measured absorbance values of the standard solutions, for example as disclosed in a paper entitled "Exploratory Study on Median-based. Robust Regression Methods for Linear Calibration in Atomic Absorption Spectrometric Analysis" by Yuzhu Hu, Jobanna Smeyers-Verbeke and Desire L Massart published in Journal of Analytical Atomic Spectroscopy, October 1989, Vol. 4, pages 605-611. In addition, it is known to use statistical techniques to assess the quality of the calibration line as this will indicate the most appropriate statistical technique for to constructing it and such a criterion is defined by Knecht and Stork in Fresenius Z. Anal. Chem, 1974, 270, 97.
While these techniques will enable a straight line to be fitted to the measured points and the quality of that line to be assessed they do not provide an easy analysis of the cause of any lack of quality, that is whether it is due to measurement inaccuracies or to trying to fit a straight line to data which in fact defines a curved calibration line.